The term perception may be used generally for mental apprehension, but in philosophy it is now normally restricted to sense perception—to the discovery, by means of the senses, of the existence and properties of the external world. Philosophers have been concerned with the analysis of perception—that is, the study of its nature and of the processes involved in it—and with its epistemological value—that is, how far, if at all, it can be regarded as a source of knowledge about the world. Their answers to these closely interrelated questions have been formulated in various theories: the commonsense theory and other kinds of direct realism, the representative or causal theory, critical realism, the sense-datum theory, and phenomenalism. This entry will be devoted to the main features of perception that underlie the various theories and that have raised philosophical problems and controversy. It will discuss both the...